NMatrix standardize_columns extension

nmatrix_standardize_columns.rb

require 'nmatrix/nmatrix'

class NMatrix
  def standardize_columns!
    cols.times do |i|
      col = self[0..-1,i]
      self[0..-1,i] = (col - col.mean[0]) / col.std[0]
    end
    self
  end

  def standardize_columns
    clone.standardize_columns!
  end
end

n = N[[1.0,3.0,4.0],[5.0,7.0,9.0],[24.0,19.0,41.0]]
n.standardize_columns
# [
#   [ -0.7324096128940435,  -0.8006407690254355,  -0.697389673941405]
#   [-0.40689422938557973, -0.32025630761017415, -0.4483219332480461]
#   [  1.1393038422796231,     1.12089707663561,   1.145711607189451]
# ]